1. Field of Invention
This invention relates to excimer lamps that emit UV radiation; more precisely, it relates to excimer lamps in which a UV-reflecting film is formed on the inner surfaces of a discharge vessel facing the discharge space.
2. Description of Related Art
Heretofore, excimer lamps have been employed as UV light sources for surface finishing when performing such processes as cleaning, ashing or coating by irradiation with UV radiation in, for example, the manufacturing of semiconductor devices, liquid crystal displays, etc.
In such an excimer lamp, the technology of installing a UV-reflecting film on the inner surface facing the discharge space of the discharge vessel has been proposed as a means of emitting highly efficient UV radiation (see, JP 3580233 B2, for example). In an excimer lamp with a structure like this one, in which a UV-reflecting film is installed on the inner surface of its discharge vessel, a light exit window for emitting UV radiation generated in the discharge space inside said discharge vessel is formed by an area where no UV-reflecting film is formed on part of the inner surface of the discharge vessel.
Moreover, in this kind of excimer lamp, when UV radiation generated inside the discharge vessel is radiated in other directions besides the direction facing the light exit window, it can be emitted from the light exit window together with the UV radiation directly radiated onto the light exit window by being reflected by the UV-reflecting film and therefore, the UV radiation can be emitted with high efficiency.
The UV-reflecting film on an excimer lamp consists of UV scattering particles having a high rate of UV scattering and these UV scattering particles have a laminated structure. Particles of such substances as silica, alumina, magnesium fluoride, calcium fluoride, lithium fluoride and magnesium oxide are employed as UV scattering particles in the UV-reflecting film.
When UV radiation hit this kind of body UV-reflecting film composed of laminated UV scattering particles, it is scattered in a direction which is different from the initial direction of the UV radiation by being refracted or reflected on the surfaces of numerous UV scattering particles.
On the other hand, silica glass is widely employed as the material for discharge vessels in lamps that emit UV radiation, such as an excimer lamp.
As shown in FIGS. 10 & 11, some types of excimer lamps are constructed in such a way that both ends are hermetically sealed and are provided with a nearly cylindrically shaped silica glass discharge vessel 20 inside of which is a discharge space S, and the outside of which are a top wall panel 21, bottom wall panel 23, side wall panels 25 and end wall panels 26 which enclose the discharge space S which contains the discharge gas. On each of the outer surfaces 21A and 23A of the facing top and bottom wall panels 21, 23, an electrode 11 and another electrode 12 are installed opposite from each other. In this excimer lamp, the UV-reflecting film 50 is formed on the inner surface 21B of top wall panel 21 on which one of the electrodes 11 is formed and also, a light exit window for emitting UV radiation generated in the discharge space S through an area where there no UV reflective coating 50 is formed on the inner surface of the discharge vessel 20 (specifically, the inner surface 23B of the bottom wall panel 23 and the inner surface 25A of the side wall panel 25. In FIG. 10, 28 is a chip tube and 29 is a flange.
In this type of excimer lamp, when high frequency voltage is applied between the electrodes 11, 12, the discharge vessel 20 and the UV-reflecting film 50 function as a dielectric body and, in discharge space S, a discharge starting point is generated on the surface facing discharge space S on the UV-reflecting film 50 and the bottom wall panel 23 facing the UV-reflecting film 50 (specifically, the surface 51 of the UV-reflecting film and the inner surface 23B of the bottom wall panel 23), and as a result, dielectric barrier discharge occurs and through this dielectric barrier discharge, excimer molecules are formed in the discharge gas and UV radiation is emitted from the light exit window comprised of the bottom wall panel 23 and the side wall panels 25 of the discharge vessel 20.
Nevertheless, as shown in FIG. 12, when the excimer lamp is turned on, there may be abnormal discharges (a) from the end area 55 of the UV-reflecting film 50 and, when the abnormal discharge occurs, a problem arises, since unevenness in illuminance occurs on the target surface of the target body, the target surface cannot be irradiated uniformly and the discharge energy consumption balance of the entire excimer lamp is destroyed.
In other words, in an excimer lamp, if there is no occurrence of abnormal discharge (a), a countless number of column-shaped discharges (hereafter referred to as “columnar arc discharges”) (b) are almost uniformly generated at an identical discharge power in the discharge space (S), but if abnormal discharge (a) occurs, the discharge energy will be consumed by this abnormal discharge (a), so around the peripheral area of the area where said abnormal discharge (a) occurs, the discharge power of the columnar arc discharge (b) is low. Therefore, in the area where abnormal discharge (a) occurs, the discharge power of the UV radiation is lower and, as a result, on the target surface of the target body, the illuminance of the area where abnormal discharge (a) occurs is lower than for other areas.
In the example in FIG. 12, in one end area 55 (right end area) of the UV-reflecting film 50 in the excimer lamp, abnormal discharge (a) occurs, and therefore, in this excimer lamp, the light intensity where end area 55 is located is less than that of other areas.
Also, a problem arises in excimer lamps when, due to this arc discharge (a), peeling occurs at the end area 55 of the UV-reflecting film 50.